1. Field of the Invention
The present invention relates to a display apparatus having a display panel on which a light-emitting element is formed for each pixel and a driving method for the display apparatus.
2. Description of the Related Art
Examples of conventionally known light-emitting element type display apparatuses, in which light-emitting elements are arrayed in a matrix and caused to emit light to execute display, are an organic EL (ElectroLuminescent) device, inorganic EL and LED (Light Emitting Diode). Especially, active matrix driving light-emitting element type display apparatuses have advantages such as high luminance, high contrast, high accuracy, low power consumption, low profile, and wide view angle. Especially, organic EL elements have received a great deal of attention.
In such a display apparatus, a plurality of scanning lines are formed on a transparent substrate. A plurality of signal lines are also formed on the substrate to run perpendicularly to the scanning lines.
A plurality of transistors are formed in each region surrounded by the scanning lines and signal lines. In addition, one light-emitting element is formed in each region.
In recent years, the light emission efficiency and color characteristic of an organic EL element have greatly increased to the degree that the light emission luminance is almost proportional to the current density. For this reason, an organic EL display apparatus having a high gray level can be designed on the basis of a predetermined standard. According to this standard, a current value necessary for an organic EL element to emit light is about several ten nA (nanoampere) to several μA (microampere) per gray level. For an organic EL element, the driving frequency must be increased as the number of pixels increases. However, when the gray level current that flows in the organic EL element is such a small current, the time constant increases due to the parasitic capacitance in the display apparatus panel. Since it is time-consuming to supply a current having a value corresponding to a desired luminance to the organic EL element, no high-speed operation can be performed. Especially, in displaying a moving image, the image quality greatly degrades. Recently, an organic EL display apparatus that controls the gray level by a current mirror has been proposed (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2001-147659).
The organic EL display apparatus described in this reference comprises an equivalent circuit 102 with current mirror shown in FIG. 7 as an equivalent circuit of a pixel. A signal current flowing in a signal line 704 is set in accordance with the size ratio of transistors 705 and 706 that constitute the current mirror, and is therefore set to be larger than a current value necessary for the organic EL element to emit light.
More specifically, in the equivalent circuit 102 with current mirror, an organic EL element 701, transistors 702 and 707, the transistors 705 and 706 that constitute the current mirror, and a capacitor 709 are arranged for each pixel. The equivalent circuit 102 with current mirror comprises a first scanning driver (not shown) that sequentially selects a first scanning line 703 of each row and a second scanning driver (not shown) that sequentially selects a second scanning line 708 of each row. First, a scanning signal that changes from low level to high level is input to the second scanning line 708 by the second scanning driver to enable a write in the n-channel transistor 707. Subsequently, a scanning signal that changes from high level to low level is input to the first scanning line 703 by the first scanning driver to enable a write in the p-channel transistor 702. A current flows to the transistor 705 and organic EL element 701 in accordance with the current flowing to the signal line 704.
The equivalent circuit 102 with current mirror described in the above reference has the following problems.
One transistor 707 is an n-channel transistor, and the other transistor 702 is a p-channel transistor. For this reason, the manufacturing process becomes complex as compared to the manufacture of single-channel transistors. In addition, since no p-channel material that effectively operates with currently used amorphous silicon has been established yet, a polysilicon must be selected.
Furthermore, in the equivalent circuit 102 with current mirror, five transistors are formed for each pixel. For this reason, the power consumption and manufacturing cost may increase, and the yield may decrease.
The equivalent circuit 102 with current mirror requires two scanning drivers. For this reason, the manufacturing cost of the equivalent circuit 102 with current mirror is high, and the scanning driver mounting area is large.